Problem: The scale on a map is 7cm : 5km. If the distance between two cities is 45km, how far apart in cm are the two cities on the map?
Answer: The scale means that every 7cm on the map corresponds to an actual distance of 5km. An actual distance of 45km is the same as 9 $\cdot$ 5km. The distance between the two cities on the map is 9 $\cdot$ 7cm, or 63cm.